Well known in the art is a stencil printing device in which a perforated stencil sheet is wound around a circumferential surface of a cylindrical printing drum with an ink supplied thereto, and in which the ink is transferred from the printing drum to a printing sheet through the perforated stencil sheet by pressing the printing sheet to the printing drum while the drum is rotated.
In a stencil printing above, it has been proposed in Japanese patent laid open publication (Kokai) No. 62-127276 that the printing density of an image to be printed on a printing sheet is variably set by variably setting a pressing force to be applied to the printing sheet against the printing drum, in accordance with the printing density information given by a means for setting printing density information, for instance, a printing density set up key provided on an operation panel.
Furthermore, since the printing density in stencil printing changes with printing speed, Japanese patent laid open publication (Kokai) No. 06-155880 has already proposed a method for implementing stencil printing at a desired density irrespective of the change in printing speed. This method comprises variably setting a pressing force applied to the printing sheet against the printing drum, in accordance with printing speed information given by a means for setting printing speed information, for instance, a printing speed set up key provided on an operation panel.
With recent diversification in quality of printing paper and originals, there is a demand on a stencil printing device which can more widely and finely control printing density. Particularly, in case stencil printing is performed using a photograph as an original, reproduction of a wide range of gradation is desired; hence, the printing density must be set properly in accordance with quality of printing paper to obtain a desired gradation range. Furthermore, in order to reproduce a desired color hue in case of multicolor printing, density of each of colors to be mixed together should be accurately controlled. Besides, in case printing is overlaid on a sheet in the same color, printed product would be messy in appearance if printing density of the second printing is different from that of the first printing. Thus, the printing densities of the first and second printing must be matched accurately.
In case of the conventional printers described above, however, even if the same printing density is set by the printing density set up key of the printer, actual printing density differs depending on printing paper because the printing density is largely influenced by quality of printing paper. Moreover, degree of pressing force that compensates for a change in printing speed to obtain the same printing density had to be determined only by experience. Thus, to obtain the same printing density in the actual printing performed at a high speed as that realized in trial printing carried out at a lower printing speed, trial printing had to be carried out repeatedly, and much experience was necessary. Needless to say, it has been furthermore difficult to accurately predict printing densities at various combinations of printing speed and pressing force.